1. Field of the Invention
The present invention relates to a piezoelectric transformer for use in any of various power supply circuits for generating high voltages and a method of manufacturing such a piezoelectric transformer, and more particularly to a laminated piezoelectric transformer comprising an elongate piezoelectric transformer body including a pair of drive regions disposed at respective opposite ends thereof and each comprising a plurality of longitudinally polarized piezoelectric layers and inner electrodes which are alternately superposed, and a longitudinally polarized electric generator disposed longitudinally centrally between the drive regions, and a method of manufacturing such a laminated piezoelectric transformer.
2. Description of the Related Art
Heretofore, it has been customary to use coiled electromagnetic transformers for generating high voltages in high-voltage power supplies for electron beam deflection units in television sets, chargers in copying machines, and other devices which need high voltages. The coiled electromagnetic transformers are of such a structure that a conductive wire is wound around a magnetic core. To achieve a high transformation ratio, it is necessary to increase the number of turns of the wire. Accordingly, it has been difficult to manufacture small-size electromagnetic transformers with high transformation ratios.
There has been proposed a piezoelectric transformer which operates on the principles of the piezoelectric effect. The proposed piezoelectric transformer is called a third-order-Rosen type piezoelectric transformer.
The boosting ratio of the piezoelectric transformer depends on the thickness of a drive region between electrodes and the length of an electric generator. In order to increase the boosting ratio, it is necessary to reduce the thickness of the drive region and increase the length of the electric generator.
Since the conventional piezoelectric transformer has a single piezoelectric layer, there is a certain limitation on efforts to reduce the thickness of the drive region between the electrodes and also on efforts to increase the length of the electric generator in view of demands for reducing the size of the piezoelectric transformer. Practically, it has been impossible to reduce the thickness of the drive region less than 0.5 mm. Another problem encountered when the thickness of the drive region is reduced is that the transformation efficiency of the piezoelectric transformer is lowered as the thickness of the drive region is reduced.
Japanese laid-open patent publication No. 224484/94 discloses a laminated piezoelectric transformer designed to eliminate the above problems. The disclosed laminated piezoelectric transformer is shown in FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the disclosed laminated piezoelectric transformer has a pair of low-impedance drive regions 91 each comprising piezoelectric layers 911 and inner electrodes 912, 913 which are alternately superposed, with outer electrodes 914, 915 disposed on respective upper and lower surfaces of each of the low-impedance drive regions 91. The outer electrodes 915 are mounted on the lower surface remotely from the outer electrodes 914 and omitted from illustration. The inner electrodes 912, 913 which are alternately exposed on opposite sides are electrically connected by outer electrodes 916, 917. The outer electrodes 917 are mounted on the side remotely from the outer electrodes 916 and omitted from illustration in FIG. 1. The outer electrodes 917 are electrically connected to an external terminal 919, and the outer electrodes 916 are electrically connected to an external terminal 920.
The low-impedance drive regions 91 between the outer electrodes are polarized transversely along their thickness. The disclosed laminated piezoelectric transformer also has a pair of high-impedance electric generators 92 with a web electrode 918 disposed therebetween. The web electrode 918 is connected to an external terminal 921. The electric generators 92 are polarized longitudinally along the length of the laminated piezoelectric transformer. When an AC voltage is applied between the external terminals 919, 920 to energize the piezoelectric transformer, a voltage is outputted between the external terminals 919, 921.
The piezoelectric transformer is produced by laminating and sintering green sheets of ceramics as the inner electrodes 912, 913 and then baking the outer electrodes 914, 915 thereon. Consequently, the thickness of the portion of the piezoelectric transformer between the outer electrodes 914, 915 may stably be reduced, and the boosting ratio thereof may be increased. Since the laminated piezoelectric structure may be increased in their overall thickness, the transformation efficiency is not unduly lowered.
Japanese laid-open patent publication No. 151677/95 reveals a laminated piezoelectric actuator wherein inner electrodes are connected through openings. FIGS. 3A through 3C of the accompanying drawings shows the revealed laminated piezoelectric actuator. The laminated piezoelectric actuator, denoted by 1100, is of a cylindrical shape and comprises ceramic piezoelectric layers each having an opening 1101 with an electrode extending to edges thereof and an opening 1102 with an electrode terminating short of edges thereof. The ceramic piezoelectric layers are stacked, and the openings 1101, 1102 are filled with an electrically conductive elastic material 1103 to electrically connect the electrodes. Each of the openings 1101, 1102 has a thickness ranging from 1 mm to 2 mm. To fill the openings 1101, 1102 with the electrically conductive elastic material 1103, the electrically conductive elastic material 1103 is pressed into the openings 1101, 1102 after the piezoelectric layers with the openings 1101, 1102 are stacked and sintered.
The conventional piezoelectric transformers described above suffer the following shortcomings:
(1) With the third-order-Rosen type piezoelectric transformer, if the boosting ratio is to be increased for producing a high output voltage from a low input voltage, then it is necessary to reduce the thickness of the drive region between the electrodes or increase the length of the electric generator. Inasmuch as the piezoelectric transformer employs a single piezoelectric layer, there is a certain limitation on efforts to reduce the thickness of the drive region between the electrodes and also on efforts to increase the length of the electric generator in view of demands for reducing the size of the piezoelectric transformer. Furthermore, as described above, when the thickness of the drive region is reduced, the transformation efficiency of the piezoelectric transformer is lowered as the thickness of the drive region is reduced.
(2) The laminated piezoelectric transformer can have its boosting ratio increased as the thickness of the drive region between the electrodes can be reduced. However, as shown in FIG. 1, the electric connection of the inner electrodes 912, 913 with the outer electrodes 916, 917 on the sides fails to provide a sufficiently high transformation efficiency. Specifically, when the piezoelectric transformer vibrates, as shown in FIGS. 2A and 2B of the accompanying drawings, it is oscillatingly displaced in directions 1003 along the thickness thereof, directions 1001 along the width thereof, and directions 1002 along the length thereof. The outer electrodes 916, 917 on the sides of the piezoelectric transformer as shown in FIGS. 1 and 2a, however, act to impede the displacement in the directions 1001, 1003, reducing the transformation efficiency.
(3) To produce the piezoelectric transformer, green sheets of ceramics are laminated and sintered as the inner electrodes 912, 913, and then the outer electrodes 914, 915 are baked thereon. As a result, the assembly is heated twice, tending to reduce the transformation efficiency of the resultant piezoelectric transformer. Furthermore, because the electrodes are formed twice, the number of fabrication steps for the piezoelectric transformer is relatively large. Since the outer electrodes are formed on the upper and lower surfaces and also on the sides of the piezoelectric transformer, the number of fabrication steps for the outer electrodes is also large.
(4) The openings in the laminated piezoelectric actuator have a relatively large diameter ranging from 1 mm to 2 mm. Therefore, the openings which are filled up with the electrically conductive elastic material are likely to impede vibrations of the piezoelectric actuator, depending on the position of the openings. Specifically, the electrically conductive elastic material that fills the openings serve as a weight load in the directions in which the piezoelectric actuator vibrates, and is liable to impede vibrations of the piezoelectric actuator. Moreover, the electrically conductive elastic material is pressed into the openings after the ceramic piezoelectric layers are stacked. If the ceramic piezoelectric layers are thinned and their number is increased for a higher boosting ratio, then the electrodes may possibly break off when the electrically conductive elastic material is pressed into the openings.